1. Field of the Invention
The present invention relates generally to a protection device for a circuit and more particularly, to a dual protection device having two units functioning independently to ensure that the circuit is protected when overloaded, overheated or shorted.
2. The Prior Arts
Electricity plays an import part in everyday life. There are innumerable applications of electricity in the world of today, from computers, home appliances, traffic, and education to entertainment. Therefore, using electricity safely is very important.
Generally speaking, a whole circuit includes a main switch which controls the ON and OFF states of the circuit. The main switch has a fuse and/or a circuit breaker to cut off the circuit and prevent the whole circuit from damage, when the circuit is overloaded, overheated or shorted.
Moreover, the whole circuit may include several sub-circuits and each sub-circuit may have its own switch to control the ON and OFF states. In order to enhance the safety of the circuit, a lot of switches for the sub-circuit are also equipped with protection function. Thus, if the main switch does not function properly or cut off the circuit in time, the switches for the sub-circuits can still protect the whole circuit from over-temperature or over-current.
In addition to the fuses and circuit breakers for the whole circuits, some electric appliances with high power consumption, such as digital processing units, electronic products or heaters, are equipped with individual protection devices to prevent them from damage when these electric appliances are overloaded, overheated or shorted. Once the individual electric appliance is overload, overheat or short, the protection device cuts off the circuit to protect the appliance and avoid damage to other appliances using the same circuit.
Referring to FIGS. 1 and 2, a conventional protection device for individual electric appliance includes a bi-metallic strip 101 which is slightly curved. The bi-metallic strip 101 bends one way if heated, and in the opposite direction if cooled off. One end of the bi-metallic strip 101 is fixed to a first terminal 102 of the electric appliance and the other end of the bi-metallic strip 101 is a free end having a first contact point 103. A second terminal 104 of the electric appliance has a second contact point 105 which is located corresponding to the first contact point 103. When the appliance is in a normal operation state, the first contact point 103 contacts with the second contact points 105 and a closed circuit is formed as shown in FIG. 1. When the circuit is overloaded, overheated or shorted, the temperature of the bi-metallic strip 101 is raised so that the bi-metallic strip 101 is bent toward the other direction and the first contact point 103 is separated from the second contact point 105. The circuit is cut off to become an open circuit and the electric appliance is then prevented from being burned.
However, the conventional bi-metallic strip has the following disadvantages. (1) When manufacturing the bi-metallic strip 101, it is difficult to maintain the precision of the thickness, curvature and configuration of the bi-metallic strip 101. Therefore, the predetermined temperature of the bi-metallic strip 101 to bend is difficult to control. (2) Some of the bimetallic strip 101 is not sensitive enough to the temperature and does not bend to cut off the circuit as expected when the circuit is overloaded. Therefore, the electric appliance and the circuit are not protected from overload, overheat or short. (3) Some of the bi-metallic strip 101 bends but does not completely separate the contact points 103 and 105. The circuit remains as a closed circuit. Thus, the appliance and the circuit are still damaged by overload. (4) Furthermore, some bi-metallic strip 101 does not separate the two contact points 103 and 105 to a sufficient distance. When the temperature slightly cools down, the bi-metallic strip 101 bends to re-connect the two contact points 103 and 105. When the temperature is raised again, the bi-metallic strip 101 bends to separate the two contact points 103 and 105. The repeated connection and separation of the circuit in a short period of time may generate sparks, which may cause fire. It also makes the electric current unstable. Thus, the electric appliance can not function properly and even be damaged.